1. Field of the Invention
The present invention relates to a method of spiking a mixed acid liquid in a reactor. More specifically, the invention relates to a method of fine-tuning spiking a mixed acid liquid in a reactor by using a computer to control a concentration of the mixed acid liquid at a target level.
2. Description of the Related Art
In semiconductor process, a mixed acid liquid is usually used to etch or wash a wafer. The mixed acid liquid used to etch the wafer includes hydrogen fluoride (HF)/ethylene glycol (EG) and hydrogen fluoride (HF)/glycerin (GLY). The mixed acid liquid used to wash the wafer includes hydrogen peroxide (H2O2)/sulfuric acid (H2SO4) and ozone (O3)/sulfuric acid (H2SO4). Such mixed acid liquid is made up of a smaller amount of highly volatile acid(s) and a larger amount of lowly volatile acid(s). The concentration of the highly volatile mixed acid liquid determines the stability of the process. In the case of etching a silicon nitride layer by using hydrogen fluoride (HF)/glycerin (GLY) in a batch type wet etching equipment, the mixed acid liquid is made up of a smaller amount of HF and a larger amount of GLY. The etching rate of silicon nitride is based on the concentration of HF. HF decreases as the process runs, because of a so-called loading effect. Meanwhile, HF is evaporated in a thermally processing tank, or is removed from the tank by exhausting. Therefore, in order to prevent HF from being decreased and thus prevent the etching rate of silicon nitride from being reduced, HF has to be supplemented to the tank to keep the process as stable as possible.
Here, “loading effect” means that active species decrease as the number of etching or washing batch increases. Therefore, in order to maintain the concentrations of the active species, the active species have to be supplemented timely, for example, after each run or every certain run.
The supplement of the active species after each run or every certain run is illustrated in FIG. 1a and FIG. 1b. HF is the active specie in the case of FIG. 1a and FIG. 1b, with HF spike timing set at four spiking strokes for each run (before or after running), and HF spike timing set at three spiking strokes every five runs respectively. 5 c.c. HF is added for each spiking stroke.
“Thermal effect” means that the active species in a reactor is evaporated by heating or removed by exhausting, thereby the concentration of the active species decreases as time elapses. Under the thermal effect, the concentration of the mixed acid liquid containing volatile components cannot be effectively controlled. Therefore, the active species has to be timely supplemented.
In order to overcome the above problems caused by the loading effect and the thermal effect, an air valve is provided under a measuring tank for spiking as shown in FIG. 2a, or a measuring pump is used for spiking as shown in FIG. 2b. In FIG. 2a, the air valve 204 is opened for several seconds after the process has proceeded for a predetermined period, to introduce HF into a HF/GLY reactor 206, so as to increase the concentration of HF. However, this method has some drawbacks. For example, there is time delay in opening/closing the air valve 204 to introduce HF. The introduced amount of HF cannot be fine tuned, resulting in undesirably high HF concentration when HF is initially added. Undesirably high HF concentration causes a sharp increase in the etching rate.
Another approach to solve the above problems is to open the air valve 204 for several seconds after or before one or more runs to supplement HF. However, as discussed above, the introduced amount of HF cannot be fine tuned, resulting in undesirably high HF concentration when HF is initially added. Therefore, the disadvantages of the prior art cannot be overcome.
In FIG. 2b, HF is introduced by using a measuring pump 208. The measuring pump 208 controls concentration more precisely than the air valve 204. However, the concentration of HF can be not kept substantially constant after several runs. Instead, the concentration of HF is increasingly higher or lower and thus makes the etching rate unstable.
In FIG. 3a, the measuring pump is set at four spiking strokes per hour, while the spike timing is not more than four spiking strokes per hour. The excess supplement increases the HF concentration in the reactor and then increases the etching rate. In FIG. 4a, the x-axis represents the lots of product, and the y-axis represents the etching rate. When the etching rate increases over a high specification limit, the products are not subject to a subsequent process and this is considered as a defect.
In FIG. 3b, the supplement timing is set at three strokes per hour. However, the required HF supplement in the operation is more than three spiking strokes per hour. The insufficient supplement decreases the HF concentration in the reactor and then decreases the etching rate. In FIG. 4b, the x-axis represents the lots of product, and the y-axis represents the etching rate. When the etching rate decreases below a low specification limit, the products are not subject to a subsequent process and this is considered as a defect.
It is the disadvantage of HF supplement by using the air valve that quantitative HF supplement cannot be achieved. Even if the measuring pump 208 controls the supplement of HF more precisely than the air valve, still the supplementary amount of HF cannot be fine tuned. The stability of etching is therefore not stable.
A monitor wafer is commonly used to test the etching amount. By using the resultant etching amount and a grouping table of FIG. 5a, an etching time and a recipe thereof are determined. This method is called a grouping run. For example, when 115 angstroms (about 10−8 cm) of etching amount of the monitor wafer is measured, the etching amount is in the range of group 1 (i.e. 110-120 angstroms) of the grouping table. When the etching amount increases to 140 angstroms, the etching time increases to 341 seconds. That is, processing recipe 11 is performed.
Another type of grouping run is to place a monitor wafer together with the products that are to be processed into the reactor. After the processing is completed, the etching rate is calculated (etching rate=etching amount/etching time). This method goes faster than the previously mentioned grouping run, because it does not need to test the monitor wafer first. A grouping table, as shown in FIG. 5b, is required for determining the etching time and the recipe program for the next lot of products.
The grouping run has been well used in a mixed acid liquid process with unstable concentration. However, the throughput of the product is significantly affected, because of the necessary step of testing the monitor wafer. Furthermore, many monitor wafers are disadvantageously wasted.
TW Patent No. 122,834, title of “Method of adjusting concentration of APM solution in a semiconductor process,” discloses that the concentration of APM solution is fine tuned by detecting the concentration of the APM solution. However, GLY is a liquid with high viscosity and a mixture of HF and GLY also has high viscosity. Therefore, detecting the concentration of solution cannot effectively and precisely control the supplement of HF.